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<a name="Defining-Mode-Iterators"></a>
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<a name="Defining-Mode-Iterators-1"></a>
<h4 class="subsubsection">17.23.1.1 Defining Mode Iterators</h4>
<a name="index-define_005fmode_005fiterator"></a>

<p>The syntax for defining a mode iterator is:
</p>
<div class="smallexample">
<pre class="smallexample">(define_mode_iterator <var>name</var> [(<var>mode1</var> &quot;<var>cond1</var>&quot;) &hellip; (<var>moden</var> &quot;<var>condn</var>&quot;)])
</pre></div>

<p>This allows subsequent <samp>.md</samp> file constructs to use the mode suffix
<code>:<var>name</var></code>.  Every construct that does so will be expanded
<var>n</var> times, once with every use of <code>:<var>name</var></code> replaced by
<code>:<var>mode1</var></code>, once with every use replaced by <code>:<var>mode2</var></code>,
and so on.  In the expansion for a particular <var>modei</var>, every
C condition will also require that <var>condi</var> be true.
</p>
<p>For example:
</p>
<div class="smallexample">
<pre class="smallexample">(define_mode_iterator P [(SI &quot;Pmode == SImode&quot;) (DI &quot;Pmode == DImode&quot;)])
</pre></div>

<p>defines a new mode suffix <code>:P</code>.  Every construct that uses
<code>:P</code> will be expanded twice, once with every <code>:P</code> replaced
by <code>:SI</code> and once with every <code>:P</code> replaced by <code>:DI</code>.
The <code>:SI</code> version will only apply if <code>Pmode == SImode</code> and
the <code>:DI</code> version will only apply if <code>Pmode == DImode</code>.
</p>
<p>As with other <samp>.md</samp> conditions, an empty string is treated
as &ldquo;always true&rdquo;.  <code>(<var>mode</var> &quot;&quot;)</code> can also be abbreviated
to <code><var>mode</var></code>.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">(define_mode_iterator GPR [SI (DI &quot;TARGET_64BIT&quot;)])
</pre></div>

<p>means that the <code>:DI</code> expansion only applies if <code>TARGET_64BIT</code>
but that the <code>:SI</code> expansion has no such constraint.
</p>
<p>Iterators are applied in the order they are defined.  This can be
significant if two iterators are used in a construct that requires
substitutions.  See <a href="Substitutions.html#Substitutions">Substitutions</a>.
</p>



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